This invention relates generally to packaging of materials, notably foodstuffs such as coffee, or foods laden with gas-producing bacteria, yeasts or molds. More particularly, it concerns a unidirectional flow pressure relief valve which allows controlled release from the package of the internally generated gases and at the same time prevents reversed-flow ingress of the ambient atmosphere through the valve.
Freshly roasted coffee is known to give off carbon dioxide gas over an extended period of time. When roasted coffee has been packaged and sealed in flexible bags fabricated of material generally impervious to passage of gases, the release of gases, notably carbon dioxide, from the packaged coffee causes distortion and bulging of the bag. This bulging can and frequently does cause rupture of the bag along the seams or in the parent material thereby exposing the contents to the ambient atmosphere. Exposure of roasted coffee to the atmosphere causes rapid deterioration in the quality of the coffee which requires, if possible, that storage be terminated and that the product be used relatively soon after exposure.
Other foods which generate gases, such as some cheeses, are similarly affected by deterioration or drying when their sealed container is ruptured.
Many valves have been developed for the release of pressure from a container when prescribed pressure limits are exceeded, but generally, these are too complex and costly to use on mass production items especially in relation to the simple, inexpensive bags used for foodstuff packaging, and in relation to the relatively low unit value of the packaged product itself.
A need for simplicity and low cost has led to the development of many inexpensive valves specifically for application to foodstuff packages such as coffee bags. These applications generally require that the packages remain substantially flat-walled during storage, that small pressure differentials cause actuation of the valve to release internally generated gases, and that the valves be self-sealing to prevent reverse flow of atmospheric gases after release of the internal pressures. U.S. Pat. No. 2,595,708 is illustrative of a vented package using a porous material communicating via an extended passage between the internal and external environments of the package. This device allows venting of food-generated gases but continuously leaves open a passage, albeit restricted, for inflow of atmospheric moisture and gases. U.S. Pat. No. 3,937,396 is illustrative of a valve for a vented package which relies on an internal pressure buildup to distort the flexible package walls to the point where a bonded seal is broken. Breaking the seal exposes an opening which communicates directly, or via an extended passage, between the interior and exterior of the package. This valve relies on the "memory" or resiliency of the material to return the distorted material to its original position after venting relieves the internal pressure. Unfortunately, the bonded seal once broken is not restored. As a consequence backleakage of external atmosphere into the package is not prevented as effectively as in the original sealed condition prior to venting and shelf life of the packaged material is shortened.